Field of the Invention
The present invention relates to an image sensor, an image capturing apparatus, and a cellular phone and, more particularly, to an image sensor including an A/D converter, and an image capturing apparatus and cellular phone that use the image sensor.
Description of the Related Art
There has been conventionally proposed an image sensor with a pixel readout circuit which includes an analog-digital converter (to be referred to as a “column A/D converter hereinafter) for each pixel column. In the mechanism of the column A/D converter, a comparator compares the level of a pixel signal with the level of a reference signal which ramps up. After that, the time from when output of the reference signal starts until the output of the comparator is inverted is counted, and the counted time is output as the digital value of the signal output, thereby performing A/D conversion.
There exist comparators, the number of which is equal to that of columns, and which are arrayed along one side or two opposite sides of a pixel region in which a plurality of pixel circuits are arranged. Reference signal lines for applying a reference voltage to the plurality of comparators are laid out along one side of the pixel region so as to be connected to the plurality of comparators, and each reference signal line is connected to one input terminal of each comparator.
To improve the image quality, there is a tendency to increase the pixel density, and the interval between the comparators is becoming small, thereby causing coupling between the output of one comparator and the pixel signal or reference signal of an adjacent column. When coupling occurs, the voltage level of the pixel signal or reference signal may vary. Especially when the voltage level of the reference signal varies, the timing at which the output of the comparator of the column is inverted may shift from the original timing when the voltage level does not vary, and thus it may be impossible to obtain a digital output corresponding to the amount of received light. A phenomenon in which the outputs of the plurality of comparators are simultaneously inverted when the voltage level of the reference signal does not vary readily occurs when an object whose brightness is uniform is captured and the voltage levels of a plurality of pixel signals (in column signal wirings) input from a plurality of photoelectric conversion elements to the plurality of comparators are equal to each other. To deal with the problem in which the voltage level of a reference signal varies, in Japanese Patent No. 4640507, the input terminals of a comparator to which a reference signal and pixel signal are input are exchanged for each set of adjacent columns. With this arrangement, since the inversion levels of the comparators of the adjacent columns are opposite to each other, the phases and magnitudes of variations of the voltage levels of the reference signals are opposite to each other, thereby reducing the influence.
In the conventional technique disclosed in Japanese Patent No. 4640507, however, it is expected that variations of the voltage levels of the reference signals of the adjacent columns cancel out each other. When, however, the pixel signal levels are slightly different from each other, the influence is limited.